Efficiently separating solids from liquids is a widespread technical problem. One of the most practical and robust methods of achieving this remains the use of a sieve, or screen, to sift the solids from the mixture of liquid and solid.
When drilling for oil and/or gas, synthetic drilling fluids, or muds, are used. As these muds are relatively expensive to manufacture, once used they are typically recovered in a process including sifting rock, shale and other debris from the mud. This involves the use of a so-called shaker which has fitted, one or more sifting screens, made up of a screen frame with one or more sheets of woven wire mesh, or screen, stretched over and secured to it. In use, the shaker vibrates the sifting screen or screens, to aid the sifting process.
In order for such sifting screens to be able to withstand the rigours of such a process, they must have a certain rigidity and be very hard-wearing. This has resulted in a design of sifting screen having a screen frame which has a plurality of reinforcing “ribs”. A common design of screen frame is rectangular comprising an outer rectangular perimeter with each side connected to its opposing side by a plurality of ribs together forming an upper face and a lower face. Such a design results in a plurality of rectangular openings. Typically the screen is attached not only to the rectangular perimeter but also to the ribs, to provide better adhesion of the screen to the frame and prolonging its lifetime. The upper face and lower face are horizontal and parallel to each other, the ribs extending downwardly between the faces orthogonally to the faces (i.e. vertically).
In view of the fact that sifting screens are man-handled into position, such screen frames have for some time been made from plastics material to reduce weight. A common design of plastics screen frame is reinforced by including a metal wire structure, embedded within the plastics rectangular perimeter and rib arrangement.
However, despite the measures taken to provide sufficient rigidity, the present inventors have found that vibratory motion typically involved in shakers is not successfully transmitted by the screen frame to the attached screen. Excessive motion of screens has been observed, known as “whipping”, which can result in erratic solids conveyancing and premature screen failure.